Focused and directed laser beams are commonly used for a variety of processes, such as drilling of blind, through and micro-vias, laser imaging, dicing of substrates and modification or customization of integrated circuits, drilling, cutting, and selective material removal and other complex machining and micro-machining operations involving materials such as metals, polymers, integrated circuits, substrates, ceramics and other materials. Such processes have become very complex, often involving the concurrent or sequential of use of single or multiple lasers or multiple types of lasers, such as visible, infra-red (IR) and ultraviolet (UV) lasers, in concurrent or sequential operations. In generally all such laser processes, however, the general object of a laser system is to controllably and reliably direct, focus and concentrate the energy of one or more laser beans to converge each beam at a desired spot or to image an apertured area of a laser beam onto the surface of an object.
A number recurring problems of conventional laser systems of the prior art, however, directly affect the reliable and controllable “pointing” of a laser beam to a desired location. The first, which is illustrated in FIGS. 1A and 1B, is often referred to as “beam wobble” or “pointing instability” and is the radial deviation of the Beam Axis 10 a Laser Beam 12 from an Optimum Centerline 14 by a Deviation Angle θ and is often related to variations in the pulse energy of the laser beam, which is often referred to as “pumping jitter”. Pointing instability is essentially inherent in both the properties of a Laser 16 itself and in the normal operations of a Laser 16, such as “pumping jitter”.
A second problem of the prior art is illustrated in FIGS. 2A and 2B and is often referred to as “thermal drift”, which again causes the Beam Axis 10 of a Laser Beam 12 to drift from an Optimum Centerline 14. Thermal drift is generally regarded as due to changes in the parameters of the Laser 16 due to changes in the laser duty cycle, heating during operation, changes in power levels of the Laser 16. It should be noted that, unlike “pointing instability” which results in an angular deviation of the Beam Axis 10 from the Optimum Centerline 14, “thermal drift” results in a linear radial drift of the Beam Axis 10 with respect to the Optimum Centerline 14. That is, the Beam Axis 10 of a Laser Beam 12 remains parallel to the axis of Optimum Centerline 14, but drifts radially away from Optimum Centerline 14.
Yet a third problem of the prior art is that of beam mode changes over time, which results in “hot spots”, or distortions of the beam profile. If the profile of the beam is non-uniform or does not have an optimum Gaussian profile, the shape of the profile cannot be subsequently shaped into the preferred “flat top” profile, which will adversely effect the quality of the processes performed by the laser system, such as micro-machining or the drilling of microvias. This problem is further compounded, of course, by pointing instabilities and thermal drift.
Effectively all lasers used for micro-machining, such as microvia drilling, exhibit pointing instabiity, thermal drift and profile distortion, and there have been many attempts to correct or at least control these problems. For example, laser systems of the prior art have attempted to correct the effects of “pointing instability” and “thermal drift” by the use of actively controlled servo-mirrors, which are controlled to redirect a laser beam so as to correct for the “pointing instability” and “thermal drift”. Such methods, however, require detecting and comparing the actual path of a beam due to pointing instability or thermal instability with the desired optimum path for the beam and controlling the servo-mirrors so as to direct the beam into the desired path. Not only are such methods complex and expensive, but they have an inherent time delay in detecting and correcting the effects of pointing instability or thermal drift, and introduce errors of their own due to mechanical and control system tolerances and have thereby not provided completely satisfactory solutions to these problems.
Other approaches of the prior art to these problems have used optical elements in the laser beam path to correct for pointing instabilities and thermal drift and to shape the beam into the optimun Gaussion and flat-top profiles for micro-machining, such as the drilling of microvias. A recurring problem, however, is that when the an optical beam shaping system is illuminated poorly, that is, either at an incident angle or with a laterally displaced beam, such as may result from pointing instabilities, thermal drift or hot spots, the optical beam shaping elements are not able to shape the laser beam into the desired profile. It will be apparent, however, that pointing instabilities and thermal drift will, in themselves, result in the beam reaching the beam shaping elements at an incident angle or with a lateral displacement, thereby resulting poor illumination of the beam shaping elements and problems in appropriate shaping of the beam profiles.
These problems arising with the use of optical elements to correct or compensate for pointing instability and thermal drift are illustrated in FIGS. 3A and 3B with respect to the use of holographic optical elements (HOEs) and standard symmetric holographic optical element (SSHOEs) employed as beam shaping elements. FIG. 3A, for example, illustrates the results of radial displacement due to thermal drift effects in the case of a Holographic Optical Element (HOE) and, in particular, with respect to a Standard Symmetric Holographic Optical Element (SSHOE) 18, or an equivalent lens. Because the SSHOE 18 is symmetric, a Laser Beam 1 2A that enters the SSHOE 18 along a Beam Axis 10A that is parallel to the HOE Axis 20 will exit the SSHOE 18 as Laser Beam 12B on Beam Axis 10B wherein Beam Axis 10B is coaxial with and a linear continuation of Beam Axis 10B. More specifically, a Laser Beam 12A entering the SSHOE 18 along a Beam Axis 10A that is parallel to but radially displaced by a distance D from the HOE Axis 20 will exit the SSHOE 18 along the same Beam Axis 10A, indicated as Beam Axis 10B, and will remain radially displaced with respect to the HOE Axis 20 by a distance D. As such, a SSHOE 18 or equivalent symmetric lens will not radially redirect the Beam Axis 10 of an entering Laser Beam 12 with respect to the HOE Axis 20 of the SSHOE 18, and thereby cannot correct for or control thermal drift effects.
Referring to FIG. 3B, a Laser Beam 12A effected by “pointing instability” will enter an Entry Face 22 of the SSHOE 18 along Beam Axis 10A having an angular deviation θ with respect to the HOE Axis 20, that is, will not be parallel with the HOE Axis 20. Because of the symmetry of a SSHOE 18 or equivalent symmetric lens, however, the Laser Beam 12B will exit the Exit Face 24 of the SSHOE 18 along a Beam Axis 10B that is the continuation of the Beam Axis 10A along which the Laser Beam 12A entered the SSHOE 18. As in the case of thermal drift, therefore, conventional SSHOEs 18 and similar symmetric lenses cannot correct for or control pointing instability and the resulting angular deviation of the Beam Axis 10.
The present invention provides a solution to these and related problems of the prior art.